A conventional surface acoustic wave (SAW) device is disclosed in Japanese Patent Laid-Open No.11-163661. FIG. 10 is an enlarged cross sectional view of a primary part of the SAW device. The SAW device includes a piezoelectric substrate 1 made of mono-crystal material, such as lithium tantalate, and interdigital transducer (IDT) electrodes 2 on the surface of the piezoelectric substrate 1. The IDT electrode 2 includes an upper layer 5 and a lower layer 3 both made of aluminum alloy containing titanium and an intermediate layer 4 made of Cu, Si, Ge, or the like for preventing aluminum alloy crystalline particles from growing and for reducing the local cell corrosion.
The IDT electrode 2 is made of the aluminum alloy at its top. Therefore, when being exposed to high-moisture atmosphere for a considerable length of time, the electrodes may cause aluminum in it to be corroded, thus declining its properties.